When my engineers are low on actual tasks, I encourage them to learn. Go read up on some new technology, play around with that game engine, read some technology magazines or books, etc. That increased knowledge is incredibly useful to the company in the long run, it makes for happier employees, and they are even more marketable. Sounds counterproductive, but I prefer more marketable employees because it means not only do I have a strong team, but now the company is far more appealing to other marketable employees when I have an opening.